Marine riser structure



Feb. 24, 1970 G. w. MORGAN 3,496,898

MARIfiE RISER STRUCTURE Filed May 15, 1968 3 Sheets-Sheet 1 FIG. I

INVENTOR. GEORGE W. MORGAN ATTORNEY Feb. 24, 1970 r .G. w. MORGAN3,496,898

' MARiNE RISER STRUCTURE Filed May 15,,3968 3 Sheets-Sheet 2 FIG. '40

- FIG. 3a

FIG. 2b

INVENTOR.

GEORGE W. MORGAN ATTORNEY Feb. 24,1970 G. w. MORGAN 3,49

' MARINE RISER STRUCTURE Filed May 15. 1968 3 Sheets-Sheet s H6. 5INVENTOR.

GEORGE W MORGAN ATTORNEY 3,496,898 MARINE RISER STRUCTURE George W.Morgan, Anaheim, Calif., assignor to North American Rockwell CorporationFiled May 15, 1968, Ser. No. 729,286 Int. Cl. B63b 35/00; E2111 43/01US. Cl. 114-0.5 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THEINVENTION Field of the invention The present invention relates to amethod and means for attaching a riser structure to a floating surfacefacility and more particularly, to such a method and means forminimizing the bending moments and other stresses in the riser structurecaused by the incompatible motions of the riser structure and thefloating surface facility.

Description of the prior art The last few years have evidenced asignificant increase in the investigation and development of thepotentials of the oceans of the world. One area of considerable activityhas been the drilling of offshore wells to locate oil and other liquidsand gases. After the offshore well is drilled, it is necessary toprovide a riser connection between the sea floor and a floating surfacefacility to conduct liquids, gases and electrical power between the seafloor and the surface facility. The surface facility may be a tanker,processing ship, barge or other floating vessel.

While it may appear that it would be a simple matter to provide areliable riser connection between the sea floor and the floating surfacefacility, such is not the case. Because of the incompatible motions ofthe riser structure and the surface facility, substantial forces areplaced on the riser structure which, if not properly compensated for,would result in the failure thereof. The principal elements of thegeneral problem relate to riser dynamics, ship dynamics, and mooringsystem dynamics. Each of these principle elements are subjected todynamic forces associated with the actual sea state, to hydrostaticforces associated with the corresponding sea depth, and to environmentaleffects which result in corrosion, abrasion, and degradation ofstructural material properties. In the riser subsystem, for example, thegeometric properties of the riser structure vary constantly with timeunder the action of the dynamic forces. As a result of the geometricchanges, the force-deflection relationships undergo various types ofnon-linear changes. The same inter-relationship of forces anddisplacements exists in the mooring subsystem as well as in the shipsubsystem. For example, geometrical changes in the ships position willeffect the upper end conditions of the riser and mooring subsystems.Changes in the mooring curvature and bottom conditions will effect themooring subsystems restraining properties which effect the shipsforce-deflection relationships, and so on.

The changing relationships between water particle motions and risermotions in a fully aroused sea form the basis of the riser designproblem. The principal problem is to permit the surface facility torespond as necessary to 3,496,898 Patented Feb. 24, 1970 i the action ofthe sea while permitting a continuous flow of liquids, gases andelectrical power through the riser. In rough seas, for example, the bowfairlead on a tanker may travel vertically in excess of feet. Inaddition, it may not be practical to constrain the horizontal motion ofthe tanker to less than 10% to 15% of the water depth.

Of the many problems which effect the development of a reliable riserconnection, the present invention is primarily concerned with theminimization of the bending stresses in the riser structure.Minimization of these bending stresses involves dynamic excitationcontrol, geometry control and riser flexural rigidity control. On thelatter point, flexural rigidity control requires a consideration for therate of change of flexural rigidity in the riser structure since highstresses are associated with immediate changes in flexural rigidity. Theriser structure, at times, will be subjected to very high rates ofacceleration and deceleration. Therefore, it is necessary to provide astructure which will tend to minimize the degree of fixidity of theriser structural attachment to the surface vessel. End fixidity of theriser results in extremely high stress levels at the point ofattachment. Therefore, the hardware attachment should decouple a largeamount of ship motions which are detrimental to the functionalperformance requirements of the riser. In addition, pitch and roll ofthe vessel, when added to the angle associated with maximum horizontalexcursion, result in excessively high bending stresses in the riser atthe point of attachment unless the riser is essentially pin connected tothe ship.

Many solutions have been suggested to eliminate the high bendingstresses developed in a riser which is fixedly connected to a surfacevessel. One proposed technique, known as a mono-mooring terminal system,which permits a moored tanker to weathervane about a single buoy loadingterminal, has been developed and is operational for multiple off-loadinglines in water depths up to approximately 100 feet. In this type ofsystem, a large cylindrical buoy acts as a floating stationary platformfor a rotating deck above. The deck supports the product piping and deckhandling gear. A center Well extends through the buoy and a hydraulicswivel mechanism is located within the center well. The rotating productpiping enables the tanker to weathervane about the buoy whilemaintaining continuous product flow.

One problem with such a configuration is that the size of the buoylimits the Water depth and the turbulence of the sea with which it maybe used. As the water depth increases and the sea becomes moreturbulent, larger and larger monoamooring terminals are required todevelop the necessary mooring stability. The result may well be amooring buoy having a size equivalent to that of the tanker. Thiscreates the problem that the tanker may engage the buoy and one or bothmay be damaged. Fouling and damaging of the hoses between the buoy andthe tanker is also a potential hazard.

Another proposed technique for eliminating the high bending stressesassociated with a riser which is rigidly connected to the surfacefacility employs a semi-fixed connection at the surface facility. Asuitable tower is fixed to the ship and supports the end of the riser atthe top thereof. Bending moment relief is provided by permitting lateralmotion and rotation of the connection point at the lower end of thetower. However, while this technique substantially eliminates thebending stresses at the lower connecting point, substantial bendingstresses still exist between the lower and upper connecting points.

SUMMARY OF THE INVENTION According to the present invention, theseproblems are solved by providing a method and means for attaching ariser structure to a floating surface facility which will minimize thebending moments and other stresses therein caused by the incompatiblemotions of the riser structure and the floating surface facility. Thepresent invention resides in providing a two-axis gimbal to support theriser tower on the surface facility so that the tower may pivot about apoint intermediate the ends thereof to minimize the bending tresses inthe rised. By providing a gimballed tower which may pivot independentlyof the surface facility, the tower and the upper end of the riserattached thereto may freely rotate so that the upper end of the theriser structure and thereby substantially minimize the bending momentsat the'upper end thereof.

It is, therefore, an object of the present invention to provide a methodand means for attaching the upper end of a riser structure to a floatingsurface facility.

It is a still further object of the present invention to such a methodand means which will minimize the bending moments and other stresses inthe riser structure caused by the incompatible motions of the riserstructure and the floating surface facility.

It is a still further object of the present invention to provide meansfor attaching the upper end of a riser structure to a floating surfacefacility, which means comprises a gimballed tower.

Still other objects, features and attendant advantages of the presentinvention will become apparent to those skilled in the art from areading of the following detailed description of the preferredembodiment constructed in accordance therewith, taken in conjunctionwith the accompanying drawings wherein like numerals designate likeparts in the several figures and wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of aportion of the ocean, a surface facility and a riser structureillustrating the relationship therebetween;

FIGURES 2a, 3a and 4a are schematic diagrams illustrating the conditionsof fixed, semi-fixed and gimballed connections, respectively, betweenthe upper end of the riser structure and the surface facility;

FIGURES 2b, 3b and 4b are a series of curves showing the bending momentsassociated with the riser structures of FIGURES 2a, 3a and 4a,respectively; and

FIGURE 5 is a simplified, cross-sectional view of a gimballed towerconstructed in accordance with the teachin gs of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawingsand, more particularly, to FIGURE 1 thereof, there is illustrated thegeneral aspect of the real-life overall problem. In FIGURE 1 it isdesired to attach a riser structure, generally designated 1, whichoriginates at the sea floor 2 to a floating surface facility 3 so as toconduct hydrocarbon products and utilities between sea floor 2 andfacility 3. Surface facility 3, which may be a tanker, processing ship,barge or other floating vessel, is moored by a plurality of mooringlines 4. The principal problem is to permit surface facility 3 torespond as necessary to the action of the sea while allowing acontinuous flow of liquids, gases and electrical power through riser 1.

Referring now to FIGURE 2a, there is shown, diagrammatically, theconnection geometry between riser 1 and surface facility 3 where theupper end 5 of riser 1 is fixedly connected to surface facility 3. Insuch cases upper end 5 of riser 1 is constrained by the motion offacility 3 and its motion is independent of the physical position ofriser 1. As shown in FIGURE 2b, such a fixed connection results in abending moment curve 6 which has a substantially high peak value 7 atthe point of connection between upper end 5 of riser 1 and vessel 3.

Referring now to FIGURE 3a, there is shown, diagrammatically, theconnection geometry under circumstances where a tower 8 for supportingupper end 5 of riser 1 is fixedly connected to surface facility 3 yetwhere moment relief is provided by permitting lateral motion of riser 1and rotation of a connecting point 9 at the base of tower 8. As shown inFIGURE 3b, such a configuration is effective to decrease the maximumvalue 7 of bending moment curve 6 and to relocate it at a pointintermediate end 5 and point 9.

Referring now to FIGURES 4a and 4b, according to the present invention,it is proposed to substantially decrease even further the maximum value7 of bending moment curve 6 by providing a configuration wherein tower 8is connected to surface facility 3 by a two-axis gimbal, generallydesignated 10, which will permit tower 8 to rotate about the two axesdefined by gimbals 10 so that the upper end 5 of riser 1 may pivotindependently of the motions of facility 3 to follow the remainingcontour of riser 1.

Referring now to FIGURE 5, there is shown a gim balled tower constructedin accordance with the teachings of the present invention. Tower 8 isconnected to surface facility 3, which is moored by lines 4 to the seafloor, by means of two-axis gimbal 10. Gimbal 10 consists of an outergimbal 11 which is rotatably connected via axles 12 and 13 to surfacefacility 3. Outer gimbal 11 is permitted to rotate about the axisdefined by axles 12 and 13 to give tower 8 a first degree of rotationalfreedom. An inner gimbal 14 is rotatably connected via axles 15 and 15',the latter not being shown in FIGURE 5, to outer gimbal 11 so as toprovide tower 8 with rotational freedom about a second orthogonal axisdefined by axles 15 and 15'.

According to the preferred embodiment, tower '8 consists of an uppersection 16 and a lower section 17. Upper section 16 has an inner surface18 which is adapted to be contacted by a pair of guide rollers 19 and 20which are connected to upper end 5 of riser structure 1. Suitabletension, generally designated T, is applied to upper end 5 of riserstructure 1 so as to eliminate the slack in riser structure 1 and tomaintain the desired stress conditions therein. In this manner, assurface facility 3 moves upwardly, an increase in the tension in riserstructure 1 can be avoided by upper end 5 thereof moving downwardly withrespect to tower 8. Guide rollers 19 and 20 roll along inner surface 18of tower 8 to guarantee the smooth motions of riser 1 within tower 8. Toprovide further support for riser structure 1 as it moves verticallywithin tower 8, a second pair of guide rollers 21 and 22 may beconnected to the lower end of upper section 16 of tower 8 immediatelyabove gimbals 11 and 14. Further support for riser structure 1 isprovided by lower section 17 of tower 8 which is connected at its upperend to inner gimbal 14 and which supports, at its lower end, a thirdpair of guide rollers 23 and 24. Through the combination of rollers19-24, riser structure 1 may freely and easily move through tower 8 inresponse to the forces on riser structure 1 caused by the motions ofship 3 and the tension at the upper end 5 thereof.

Gimbals 11 and 14 provide tower 8 with two degrees of rotational freedomto thereby substantially minimize the bending moments and other stressesin riser structure 1. To prevent excessive excursions of tower 8 and toprovide damping of the motions of tower 8 under conditions of highacceleration and deceleration, tower 8 may be connected to a suitablecompensating damping system via lines 25 and 26, which also may belocked firmly in the vessel transport mode to support the tower prior toconnection with a riser system. In addition, a suitable swivel 27 may beconnected to upper end 5 of riser 1 to eliminate torsion stresses inriser 1 as vessel 3 rotates.

While the invention has been described with respect to a preferredphysical embodiment constructed in accordance therewith, it will beapparent to those skilled in the art that vario s mod fications andimprovements 6 may be made without departing from the scope and 3. Theinvention according to claim 2 wherein said spirit of the invention.second pair of guide rollers is attached to the lower I claim: portionof said tower. 1. Means on a floating surface facility for supporting 4,The invention according to claim 3 and further the upper end of a riserstructure originating at the ocean 5 omprising: floor comprising incombination; a third pair of rollers connected to the upper portiontwo-axis gimbal means operatively Conneflcd to said of said riser tofurther guide said riser structure.

surface facility; '3' a tower means having an upper and a lower sectionReferences Cited connected to said gimbal means and adapted to re- 10ceive said riser structure; UNITED STATES PATENTS a pair of guiderollers connected to said upper section 3,419,090 12/1963 V n D m,

of said tower and another pair of rollers connected 3,177,954 4/1965.Rand 175-7 to said lower section of said tower in contact with 3,341,3989/ 1967 Nicolson 175-7 said riser structure to guide the movement of 153 390 54 7/1968 B n said riser structure within said tower so as to minimize bending stresses on said riser. TRYGVE M. BLIX Pri y Exammef 2. Theinvention according to claim 1 wherein the ,f

. ;U.S. Cl. X.R.

upper port on of said tower is connected on one side of said girnbalmeans and the lower portion of said tower 2o 7 is connected to the otherside of said gimbal.

UNITED STATES PATENT OFFICE (569) CERTIFICATE OF CORRECTION Patent NO.Dated Februa 21; 1

Inventor-(s) George Morgan It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

COLUMN 3:

In line 7, "rised" should be --riser--.

In line ll, following "riser" and. before "structure" insert thefollowing:

--ca.n position itself to conform to the remainder of the riser-- Line16, after "to" insert --provide--.

SIGNED AND SEALED Ame Edwardu l'hldlor, 1:. m1 3. v I All E. m. Am:Officer commissioner or PM

